Electrochemical capacitors using a nonaqueous electrolyte are advantageous in that the withstand voltage can be increased, and thus the energy density can be increased as compared with electrochemical capacitors using an aqueous electrolyte. They have been rapidly spread as backup power sources for household electronic devices. Particularly, in those electrochemical capacitors used for electric power systems such as electric vehicles, hybrid vehicles and electric power storage having an electrostatic capacity of 50 F or higher, which have attracted attention in recent years, nonaqueous electrolyte-based ones are suited.
A nonaqueous electrolyte comprising a solution of a cyclic amidinium salt in propylene carbonate as a solvent has been put into practical use for electrochemical capacitors (e.g. WO 95/15572).